In power-line communications (PLC), utility power lines, especially the high-voltage (HV, 60 kVAC and up) and medium-voltage (MV, 4–35 kVAC) power lines, are used as a transmission medium. The MV power lines are generally used to power the primaries of distribution transformers feeding electric power to homes and businesses. It is advantageous to convey communication signals in radio frequencies (RF).
A typical scenario in PLC is shown in FIG. 1. As shown, a main power line L1 and a number of other power lines L2, L3, L4 branching off from L1 are used to carry the RF communication signals. A server 10 is used at a distribution center to receive multimedia information from service providers and to send the information to a plurality of customers downstream. The server 10 uses an RF coupler 12 and an associated distribution modem 11 to broadcast the RF communication signals on power line L1 so that customers can receive the signals using their customer premise equipment (CPE). For example, CPE 20 and CPE 30 acquire the RF signals from L1 via RF couplers 22, 32 and associated modems 21, 31, while CPE 40 acquires the RF signals from L3 via an RF coupler 42 and an associated modem 41, and so on. On the upstream direction, customers can use their CPE to send request data to the server via the same couplers and modems.
It is known that RF signals are attenuated considerably as they are transmitted along the power line. As a result, a CPE located too far from the server 10 may not be able to receive usable RF signals. For example, while CPE 20 may be able to receive good signals from the server 10, CPEs 30, 40 and 50 may not. Thus, it is necessary to provide a plurality of repeaters 72, 74, etc. along the power lines to make it possible for CPE 30, 40 and 50 to receive the communication signals.
It should be noted that although a connection is shown from, for instance, server 10 to distribution modem 11, this connection may be via a wireless radio frequency link, e.g., according to IEEE specification 802.11x (where x=a, b, c, . . . , etc) or via a fiber optic link, etc. Such connections and methods can also be used from each of the CPEs 20, 30, 40, 50, etc. and their corresponding modems 21, 31, 41, 51, etc.
Similarly the connection from distribution modem 11 and RF coupler 12 and from each modem 21, 31, 41, 51, etc. to corresponding RF couplers 22, 32, 42, 52, etc. can be electrical (voltaic), optical or wireless.
In general, it is desirable that any server or CPE not have any physical connection (voltaic or optical fiber) to its corresponding modem if the corresponding modem is voltaically connected to its corresponding RF coupler. This general design goal is to eliminate any possible failure mode where MV voltages can be brought in contact with CPEs or servers.
When a repeater receives communication signals conveyed from the upstream direction via a power line, it is designed to repeat the communication signals so that the CPE in the downstream can receive useful RF signals. These repeated signals will also travel upstream along the same power line. When there are many repeaters along the same power line repeating the same communication signals, there will be significant interference among the repeated signals because of the delay in each repeater and the overlap of signals. In general, a repeater is needed at a location when the communication signals have been attenuated significantly but are still useful. For example, the distance X between two adjacent repeaters can be the length of the power line segment such that the amplitude or the strength of the signals has reduced to 1/e after the signals traverse such length. As shown in FIG. 2, three repeaters 72, 74 and 76 are implemented on the power line L1 for RF signal repeating. As shown, repeater 72 receives RF signal S0 from Server 10 via RF coupler 12 and transmits RF signal S2. Similarly, repeater 74 receives signals S2 and transmits signal S4. Assuming that the signal strength of the signals transmitted by RF coupler 12 and the signal strength of all repeaters is substantially the same, then the signal S0, as received by repeater 72, is substantially the same as the signal S4 as received by repeater 72. Additionally, repeater 72 also receives signal S6′ transmitted upstream by repeater 76. Because signal S6′ has traveled an additional distance X, its strength is smaller than either S0 or S4 by a factor of (1/e). Thus, interference on the received signal S0 at repeater 72 mainly results from the upstream signal S4.
It would be advantageous and desirable to provide a method of RF signal repeating wherein interference in the repeated signals is greatly reduced.